1. Field of Invention
The present invention relates to a patterning method for fabricating integrated circuit. More particularly, the present invention relates to a patterning method capable of minimizing some limitations in photolithographic and etching processes.
2. Description of Related Art
As dimensions of semiconductor devices continue to shrink, the demand for resolution goes up correspondingly. Because the resolution of a photolithographic process is dependent upon wavelength of a light source used in the photo-exposure, the mask patterns obtained by conducting a photolithographic (or together with an etching) process must be separated from each other by a minimum distance. Furthermore, if the mask is used as an etching mask, gap or opening dimension in the etching mask can hardly be reduced without some adverse effect on the underlying layer waiting to be etched.
At present, the photolithographic process for patterning a photoresist layer is set such that gaps or openings in the photoresist layer have a minimum aspect ratio of 3:1. However, the photoresist layer must also have a thickness sufficient to resist etching. Hence, miniaturization of device can hardly be achieved by reducing thickness of the photoresist layer directly.
To reduce thickness of photoresist layer, a hard mask layer fabricated using a material of higher etch resistance is often employed as etching mask. In other words, the method includes using a photoresist layer to pattern a hard mask layer and then using the hard mask layer as an etching mask to pattern a material layer underneath. Because of etching selectivity between the hard mask layer and the underlying material layer, a hard mask layer with lesser thickness can be used. This relaxes the thickness requirement of photoresist layer and hence eases up some of the limitations in a photolithographic process. Yet, this method has some serious drawbacks. First, material forming the hard mask layer must be specifically selected in accordance with the material properties of the material layer to be patterned. Hence, a different hard mask layer must be used to pattern a material layer made from a different material and hence the complexity of processing design is increased. Secondly, because the hard mask layer and the photoresist layer must be made from two different materials, the photoresist layer and the hard mask layer must be removed in at least two separate steps and hence the complexity the fabrication is more complex and, thus, more costly.